NIRS, National Institute of Radiological Sciences has just started to monitor the performance of accelerator with parameter of Availability, Failure Rate and Repair time as function of Quality Management System.All troubles that occurred in a month are listed with cause of trouble, defective part and downtime to calculate above parameters.Those parameters are graphed to monitor the trend. Also, defective parts are categorized and graphed for more breakdown information.The quality review meeting is held every month. If a parameter of trend shows low quality, cause would be investigated and corrective action would be discussed in the meeting.This poster will explain how we decide the parameters as a reliability index and how we use those indexes, with the cycle of Monitor-Analysis-Correction- Evaluation that maintains and/or improves the accelerator reliability.The 5th Accelerator Reliability Workshop (ARW 2015)